The effect of variation of the rate of cooling on melt-crystallization of random copolymers of isotactic polypropylene with low amount of either ethylene or 1-butene is evaluated using X-ray techniques, atomic force microscopy, and calorimetry. Slow non-isothermal melt-crystallization mainly results in formation of monoclinic alpha-crystals of lamellar geometry. The presence of comonomers leads to a decrease of the degree of crystallinity, and of the thickness of lamellae. These changes are proportional to the concentration of co-units, and are more distinct in propylene-ethylene copolymers. Rapid cooling of propylene-1-butene copolymers leads to formation of non-lamellar mesomorphic domains, independent of the concentration of co-units within the investigated range of concentrations. In quenched propylene-ethylene copolymers, in contrast, the formation of mesomorphic structure partially is replaced by formation of monoclinic alpha-crystals of still non-lamellar geometry.